From Why aren’t my margins working in Outlook.com? :
Outlook.com no longer supports the following CSS properties: margin,
margin-top, margin-right, margin-bottom, margin-left.
Replacing margin with padding on paragraph tags works well in
Outlook.com, but Microsoft’s desktop clients didn’t fare so well:
Outlook 2007/2010/2013 don’t support padding on block level elements.
So margins work in the desktop version, but padding does not.
And in Outlook.com padding works, but not margins.
A margins or padding solution is therefore only possible if you and your receivers use only one of the Outlook flavors.
In extremity, the article recommends using tables as the all-around solution :
Use <TD>
and swap margin for padding
Placing each block of text or image inside its very own <td>
cell —
along with inline CSS padding — nets the best results across major
email clients. While web development purists are rightfully hesitant
to sully their emails with non-semantic markup, this method has worked
well for us over the years. And while padding doesn’t work quite the
same way as margin, it gets the job done. Just be especially careful
when using borders, since you’ll need to play around with nesting
tables to get your desired effect while relying solely on padding.